There is an urgent need to address the disproportionate rates of HIV/AIDS among African Americans in the United States, particularly African American females. It has been postulated that the intersection of drug use and sexual behavior among adolescents exacerbates their risk for HIV. Using a 3 year prospective cohort study of urban African American girls, ages 14-19, in Baltimore, MD, this study proposes to examine 2 pathways from marijuana use to risk for HIV within adolescent romantic relationships. We will first test the hypothesis that girls' recent marijuana use is an independent predictor of incident STI in this population of adolescent girls. Second, we will address the disinhibition pathway by testing whether marijuana use at the time sex is associated with consistent condom use. Finally, a second pathway leading from girls' recent marijuana use to incident STI will be examined. Specifically, we will test whether partner characteristics (i.e. older age), mediate the association between girl's marijuana use and STI outcome and determine whether partner marijuana use confounds the association between girl's marijuana use and STI. An improved understanding will contribute to the development of more effective HIV prevention programs.